A switched-mode power converter (also referred to as a “power converter”) is an electronic power processing circuit that converts an input voltage waveform into an output voltage waveform. The waveforms are typically, but not necessarily, dc waveforms, controlled by periodically switching power switches or switches coupled to an inductive circuit element. The switches are generally controlled with a conduction period “D” referred to as a “duty cycle.” The duty cycle is a ratio represented by the conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent).
Feedback controllers associated with power converters manage an operation thereof by controlling the conduction period of a switch employed therein. Generally, a feedback controller is coupled to an output of a power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”) to regulate a characteristic (e.g., an output characteristic) of the power converter such as an output voltage. A switched-mode power converter typically receives a dc input voltage Vin from a source of electrical power at input nodes thereof and provides a regulated output voltage Vout at output nodes thereof to power, for instance, a microprocessor coupled to the output nodes of the power converter.
Advances in microprocessors and other electronic technologies impose challenges in the design of the power supplies required to meet increasingly stringent power requirements thereof. In order to deliver a highly accurate supply voltage to the microprocessors, it is often necessary to place a voltage regulator module in the form of a dedicated dc-dc converter in close proximity thereto. The stringent regulation requirements and high load fluctuations exhibited by the microprocessors are forcing power converters to operate at ever higher switching frequencies to, among other benefits, reduce the size of the power converter and increase the ability of the power converter to respond to the load fluctuations. The high frequency operation, however, can have a detrimental effect on the efficiency of the power converter, as a significant amount of power is necessary to drive the control terminals of the switches such as the gate terminals of metal-oxide semiconductor field-effect transistors (“MOSFETs”) at higher frequencies.
Additionally, the introduction of semiconductor devices employable as switches based on group III-V semiconductor materials such as gallium arsenide (“GaAs”), indium-phosphide (“InP”), or indium gallium arsenide (“InGaAs”), to cite just a few examples, further lead to the design of power converters capable of switching at higher frequencies without incurring the detrimental gate drive losses of silicon based devices. For general information on group III-V semiconductor devices and, in particular, gallium arsenide devices, see Fundamentals of III-V Devices, by W. Liu, published by John Wiley and Sons (1999), and Modern GaAs Processing Methods, by R. Williams, published by Artech House (1990), which are incorporated herein by reference.
The lack of a native oxide for many of the group III-V semiconductor devices makes the construction of a true enhancement mode MOSFET difficult. Consequently, group III-V semiconductor devices are often fabricated as depletion mode switches, which exhibit a finite, and typically low resistance with zero volts applied to the gate terminal thereof. Fully turning off the depletion mode switch may require the application of a non-zero signal such as a negative signal (e.g., a negative gate-to-source voltage) thereto. While the application of a negative gate-to-source voltage is somewhat straightforward during steady state operation of the power converter employing such a switch, the requirement for a negative gate-to-source voltage may be problematic during non-steady state operations of the power converter or when the converter and switch are subject to transient conditions. The transient conditions typically occur during startup and shut down of the power converter, during fault conditions or protection periods of operation, or whenever the switch(es) of the power converter need to be configured off with input and/or output voltages applied thereto, to name a few.
A conventional enhancement mode MOSFET does not typically require special care during some or all of the non-steady state operations described above because the enhancement mode MOSFET exhibits a high resistance and is typically off (i.e., disabled to conduct) with the application of zero gate-to-source voltages, and a positive voltage is typically required to turn the switch on. As a normally off switch, an input voltage of the power converter may be safely applied to the enhancement mode MOSFETs (again, a high impedance switch) without detrimental effects thereto or to the power converter. A power converter employing depletion mode switches, on the other hand, may exhibit low input impedance as the input voltage is applied for the reasons discussed above. Special circuitry and/or procedures may be necessary to prevent excessive current flow during the non-steady state operations of the power converter.
Accordingly, what is needed in the art is a circuit and method of operating a power converter that employs depletion mode switches taking into account non-steady state operations thereof and adapted to apply a suitable signal to turn the switch(es) off (or disable the switch(es)) at the appropriate times to prevent a detrimental impact on the power converter and devices (including the switch(es)) therein, that overcomes the deficiencies in the prior art.